Conventional thermosetting coatings contain a fluidity-controlling agent in order to control the fluidity of coating and give a coating film of smooth surface and also to substantially eliminate the sagging of coating applied on a vertical plane. As the fluidity-controlling agent, various types are known. Generally and widely used are, for example, inorganic additives such as AEROSIL, Bentone and the like; polyamide compounds such as Disparlon (trade name, a product of Kusumoto Chemicals, Ltd.) and the like; diurea compounds obtained by the reaction of a diisocyanate compound and a primary amine; and finely divided gelled polymers.
These fluidity-controlling agents have influences on the rheology and physical properties of coating composition and, as a result, can improve the spraying efficiency of coating, the sagging-preventability of coating film, the pattern controllability of metallic pigment, etc. On the other hand, the fluidity-controlling agents have had problems in that they reduce the finish appearance (e.g. luster) of coating film, the intercoat adhesion when a plurality of coatings are applied in layers, and the water resistance of coating film.
In order to alleviate the above problems when a conventional fluidity-controlling agent is used, coating methods were proposed which comprises ejecting a curable coating composition from a spray gun and spray-coating the ejected composition while applying an active energy beam thereto. In these methods, the curable coating composition has a low viscosity right after injection but has a high viscosity when coated on a material to be coated, whereby sagging of coating from the coated material can be prevented.
For example, in JP-A-6-65523 is disclosed a coating method which comprises, in coating, on a material to be coated, a high-solid coating containing an acrylic resin, a heat-crosslinking agent, a photopolymerizing monomer (which has a double bond in the molecule and can be polymerized by an electromagnetic wave), a photopolymerization initiator and an organic solvent, ejecting the high-solid coating from a spray gun and spray-coating the injected coating while applying a given electromagnetic wave to the coating.
Also, in JP-A-7-70471 is disclosed a coating method which comprises spraying, on a material to be coated, a high-solid coating containing a macromonomer having an ethylenically unsaturated bond at one end and a photopolymerization initiator, while applying an ultraviolet light to the coating particles formed by spraying and flying in the air.
These methods can certainly prevent sagging. They, however, have a problem of no applicability as a top clear for automobiles, for the following reason. In the above methods, since a photo-induced radical polymerization reaction is utilized, the polymerization reaction of double bonds is easily hinderd by the presence of oxygen; consequently, the double bonds remain in the coating film formed, which tends to allow the film to have various defects, for example, reduced weatherability and yellowing.